It has been known that in image display devices such as television receivers, video projectors and the like, the quality of an image is degraded by the occurrence of flares. A flare refers to a blurring phenomenon which is caused at edge portions (e.g., a border between white and black areas) where there is a large difference in brightness in the displayed image, by light from a bright area leaking into an dark area due to reflection and/or the scattering of light through the lenses and illuminated surfaces of a projection tube or an image-receiving tube (see paragraphs [0002] to [0005] and FIG. 17 in Japanese Patent Application Laid-open 2002-290772).
FIG. 1 is a schematic diagram showing one example of an original image of a picture to be projected by a projector. This original image has rectangular white area WT in the center and black area BL around white area WT with their boundary or edge portion ED having a large difference in brightness. Shown at the bottom in FIG. 1 is a video signal (luminance signal) in the horizontal direction around the center of the original image. When this original image is projected on a screen by a projector, light in white area WT leaks into black area BL so that blurring appears at edge portion ED. This blurring is the flare which degrades image quality.
In order to eliminate such flares as above, it is a common practice to perform digital signal processing for correcting the blurs at the edges of video signals to be input to the projector. FIG. 2 is a conceptual diagram of a flare correction. In FIG. 2, sub FIG. 2(a) is a waveform chart showing a video signal of an original image; sub FIG. 2(b) is a luminance distribution chart of a screen image that is displayed based on the video signal of sub FIG. 2(a); sub FIG. 2(c) is a waveform chart of a signal that is obtained from the video signal of sub FIG. 2(a) after flare correction; and sub FIG. 2(d) is a luminance distribution chart of a screen image that is displayed based on the video signal after flare correction shown in sub FIG. 2(c). Here, the video signal of sub FIG. 2(a) corresponds to the video signal of the original image shown in FIG. 1.
The screen image projected based on the video signal of sub FIG. 2(a) by a projector forms an image with its edge portions rounded as shown in sub FIG. 2(b) due to the occurrence of flare. In order to compensate for flare in this image, the leading and trailing edges of the video signal in sub FIG. 2(a) need to be corrected (conversely corrected) in accordance with the deformation of the edge portions shown in sub FIG. 2(b) or by making a correction so as to make the edges distinctive. With this correction, as shown in sub FIG. 2(b), a screen image without edge portions rounded can be obtained.
As an apparatus capable of making flare correction as above, there is an image quality improving apparatus as described in Japanese Patent Application Laid-open 2006-157228 (see FIG. 4). FIG. 3 shows a configuration of the image quality improving apparatus.
Referring to FIG. 3, the image quality improving apparatus includes delay compensation circuit 141, 2-dimensional low-pass filter (LPF) circuit 142, delay compensation circuit 143, subtractor circuit 144, amplifier circuit 145 and adder circuit 146. A luminance (Y) signal is supplied to delay compensation circuit 141 and 2-dimensional LPF circuit 142 while a chromaticity (C) signal is supplied to delay compensation circuit 143.
Delay compensation circuit 141 delays the input Y signal by the amount of time required for the process in 2-dimensional LPF circuit 142. The output from delay compensation circuit 141 is supplied to one input terminal of subtractor circuit 144 and to one input terminal of adder circuit 146. Two-dimensional LPF circuit 142 is a filter for removing high-frequency components (edge components) of the input Y-signal, and its output is supplied to the other input terminal of subtractor circuit 144. The output from 2-dimensional LPF circuit 142 presents the Y-signal with its edge portions rounded.
Subtractor circuit 144 subtracts the Y-signal supplied from 2-dimensional LPF circuit 142 from the Y-signal supplied from delay compensation circuit 141. The output from subtractor circuit 144 presents a signal of high-frequency components (edge components) that were extracted from the Y-signal. The edge component signal output from subtractor circuit 144 is supplied to the other input terminal of adder circuit 146 via amplifier circuit 145. Amplifier circuit 145 amplifies the edge component signal supplied from subtractor circuit 144. Adder circuit 146 adds the edge component signal that was amplified by amplifier circuit 145 to the Y-signal supplied from delay compensation circuit 141. The output from adder circuit 146 presents a signal that is obtained by emphasizing the input Y-signal at its edges and that corresponds to the signal shown in sub FIG. 2(c).
In the above image quality improving apparatus, 2-dimensional LP F circuit 142 can be constructed of a recursive filter (see Japanese Patent Application Laid-open 2006-157228 and FIG. 1 in Japanese Patent Application Laid-open S61-184059). In this case, in order to improve the accuracy of the 2-dimensional LPF, a time axis inverting process (field inverting process) is often used.
Further, recent image display devices are adapted to be able to perform a displaying process that supports a plurality of video signals having images different image sizes (the aspect ratio or resolution) that are to be displayed. In such an image display device, it is not preferable that flare correction processing circuits be provided in correspondence with input image signals whose image sizes that are to be displayed are different because this entails enlargement of the circuit scale. According to the apparatus in which the flare correcting process is effected after a process for modifying the input video signal so as to modify the size of the displayed image to a fixed image size that is designated for the display device, it is possible for a single flare correcting circuit to support input video signals of different image sizes, and the circuit configuration can also be made simple.
Further, in an image display apparatus such as a projector, the shape of the projected image deforms depending on the angle at which the image is projected. To deal with this, there is also a proposal of an apparatus which makes correction of geometric distortion such as trapezoidal correction to the video signal by modifying the shape of the displayed image against the deformation of the projected image (see Japanese Patent Application Laid-open 2005-266042).